Alternative to Contax G 45mm F/2

Yes.They are nice,sharp,small and cheap lenses.You may also consider MD/MC W. Rokkor 35/2.8 either 55mm of 49mm (smaller lens) filter thread.

Out of the rokkor stable other very good sompact and under 300USD lenses (35 to 50mm fl):

Zeiss Tessar 45/2.8 C/Y
Industar 3.5/50
Nikon 50/1.8 Series E
Nikon 50/2 AI
Nikon 50/1.8 AI/S or 50/1.4 AIS
Zeiss Planar 50/1.7 C/Y
Yashica ML 50/2 or 50/1.7 or 50/1.4
Yashical ML 35/2.8
SMC Pentax M 50/2 or 50/1.7 or 50/1.4
SMC Pentax M 35/2.8
Canon FD 50/1.8 or 50/1.4
Zeiss Jena Flektogon 35/2.4
Zeiss Jena Tessar 50/2.8
Zeiss Jena Pancolar 50/1.8

....to point out the most obious choices.